1. Line an 8-inch square baking dish with parchment, leaving a 2-inch overhang on two sides.

2. Melt butter in a nonstick skillet over medium heat. Add ricotta and stir with a rubber spatula until butter and ricotta are incorporated. Add milk powder and sugar; mix well to combine. Bring to a gentle simmer and cook, stirring often, until mixture is very thick and begins to form a dough that moves together in a single mass when stirred, 15 to 20 minutes. Stir in cardamom and pistachios. Transfer mixture to prepared dish and pat into an even 3/4-inch thickness (you will only use about 60 percent of dish's surface area). Let cool to room temperature. Lift barfi from pan using parchment; cut into diamonds. Garnish with gold leaf, if desired, and more pistachios. Barfi can be stored, covered, in refrigerator up to 1 week.